A storage is computer-readable media capable of storing data in blocks. Storages face a myriad of threats to the data they store and to their smooth and continuous operation. In order to mitigate these threats, a backup of the data in a storage may be created at a particular point in time to enable the restoration of the data at some future time. Such a restoration may become desirable, for example, if the storage experiences corruption of its stored data, if the storage becomes unavailable, or if a user wishes to create a second identical storage.
A storage is typically logically divided into a finite number of fixed-length blocks. A storage also typically includes a file system which tracks the locations of the blocks that are allocated to each file that is stored in the storage. The file system also tracks the blocks that are not allocated to any file. The file system generally tracks allocated and unallocated blocks using specialized data structures, referred to as file system metadata. File system metadata is also stored in designated blocks in the storage.
Various techniques exist for backing up a source storage. One common technique involves backing up individual files stored in the source storage on a per-file basis. This technique is often referred to as file backup. File backup uses the file system of the source storage as a starting point and performs a backup by writing the files to a destination storage. Using this approach, individual files are backed up if they have been modified since the previous backup. File backup may be useful for finding and restoring a few lost or corrupted files. However, file backup may also include significant overhead in the form of bandwidth and logical overhead because file backup requires the tracking and storing of information about where each file exists within the file system of the source storage and the destination storage.
Another common technique for backing up a source storage ignores the locations of individual files stored in the source storage and instead simply backs up all allocated blocks stored in the source storage. This technique is often referred to as image backup because the backup generally contains or represents an image, or copy, of the entire allocated contents of the source storage. Using this approach, individual allocated blocks are backed up if they have been modified since the previous backup. Because image backup backs up all allocated blocks of the source storage, image backup backs up both the blocks that make up the files stored in the source storage as well as the blocks that make up the file system metadata. Also, because image backup backs up all allocated blocks rather than individual files, this approach does not necessarily need to be aware of the file system metadata or the files stored in the source storage, beyond utilizing minimal knowledge of the file system metadata in order to only back up allocated blocks since unallocated blocks are not generally backed up.
An image backup can be relatively fast compared to file backup because reliance on the file system is minimized. An image backup can also be relatively fast compared to a file backup because seeking is reduced. In particular, during an image backup, blocks are generally read sequentially with relatively limited seeking. In contrast, during a file backup, blocks that make up individual files may be scattered, resulting in relatively extensive seeking.
A source storage may be initially backed up using an image backup operation to create a base backup and then in successive image backup operations, incremental backups of the source storage may be created. A new incremental backup may include only those blocks of the source storage that were changed subsequent to the creation of the most recent backup but prior to the creation of the new incremental backup. In order to easily back up only changed blocks during the creation of an incremental backup, it can be useful to incrementally track which blocks are changed between image backup operations instead of determining which blocks are changed by performing a full compare of every block in the source storage with corresponding blocks in base and incremental backups that were previously created.
One common problem that is encountered during successive image backup operations is the difficulty of reliably tracking incremental changes prior to the creation of each incremental backup. For example, incremental changes are typically tracked in volatile memory in order to avoid the performance degradation that would be caused by continually writing incremental changes to a nonvolatile storage. When a source system is gracefully shut down, the incremental changes tracking in volatile memory can be written once to a nonvolatile storage, and then read once from the nonvolatile storage and back into the volatile memory upon a graceful reboot. However, if the source system experiences an ungraceful loss of power or other crash, the incremental changes tracked in volatile memory are lost, and the only way to determine which blocks changed is to perform a full compare of every block in the source storage with corresponding blocks in base and incremental backups that were previously created. This full compare is time-intensive and resource-intensive.
The subject matter claimed herein is not limited to embodiments that solve any disadvantages or that operate only in environments such as those described above. Rather, this background is only provided to illustrate one example technology area where some embodiments described herein may be practiced.